a)you can only buy property later, i think its when you unlock trevor,
b) you can try :),what i did was come to the door, and pul the gun outside and aim at the own, then get out of sight as his shotgun will destroy you, if you want his cool shotgun before the game unlocks, call the cops and let them handle him,cos if you kill him, he will not drop he gun, then shoot the cashier,and grab the bags
c)stock open up with story,although right at the begining with franklin there were more weapons to buy then later with him,problably an oversight.
